Dynamic control of blade pitch angle is used on turbines as part of an overall control system to regulate rotor speed and torque. Techniques are also employed to adjust blades to mitigate a variety of impulse and fatigue loads in response to the time varying flow velocity conditions in the field of the swept rotor area. Changes in fluid density, as well as factors relating to turbine geometry are also considered in turbine control and load mitigation approaches.
Developments in measurement and control techniques have resulted in increased dynamic response capabilities with respect to changing the power and rotor torque produced by the turbine blades. Modern turbines using blade pitch control continue to benefit from increasing dynamic response capabilities from blade pitch actuators since increased response capability enables more complete optimization of turbine power regulation and improves the effectiveness of the mitigation of various impulse and fatigue loads.
In the case of wind turbines, blade pitch controls serve a critical safety function by providing a means to stop the turbine by moving the blades to act as an aerodynamic brake. Otherwise the turbine cannot be stopped and such loss of control can lead to catastrophic failure of the machine, and potentially pose a safety hazard to life and property. For reasons of safety, the pitch system design must be able to sustain any plausible single point failure or common mode failure and have no more than one blade fail to reach an aerodynamic “safe”, or feather, position as a result.
FIG. 1 is a schematic block diagram of a prior art DC blade control system. To satisfy this safety requirement, it is known to use electrical pitch systems with DC motors for blade pitch actuation and control using mechanical brush commutation. In these designs, a simplified example of which is shown in FIG. 1, power-switching electronics, such as an IGBT or MOSFET H-Bridge or totem poles, are used to regulate voltage or current to the motor. Blade position is measured and controlled in one of several ways by a combination of feedback sensors and electronic control processors and software. The intent is that these controls, in combination with the power switching elements that regulate motor speed and torque, result in the blade having the desired states of motion at each instant in time.
Typically, a separate, fail-safe, safety system is activated in the event of a fault in the electronic controls of the pitch system or turbine control system, preventing the continuing operation of the systems. For example, the safety system connects stored energy contained in a DC power source directly to the DC motors by electromechanical means. The motor runs to cause the blade to travel in the direction of, and stop at, the aerodynamic feather position. There is one separate and independent safety system for each turbine blade. The stopping position is controlled by position indicating switches that are mechanically linked between the blade and the motor. These limit switches electromechanically disconnect the stored energy from the pitch motor and releases power from the failsafe brake to stop the blade pitching motion.
There are at least two limitations of the mechanically commutated DC motor solution. With any motor technology the relationship between torque and inertia defines the maximum acceleration capability of the motor and the whole system it drives. With DC motors the inertia tends to be high in relation to the torque and hence the system acceleration is limited substantially by the motor inertia itself. This acceleration limit sets the overall dynamic response for the pitch actuator.
Mechanical brushes in DC motors are a maintenance item and may need to be replaced from time to time. Increasing demands on pitch actuator dynamic response tends to work against long service life for the motor brushes and commutator.
AC systems overcome one or both of the above limitations in DC mechanically commutated systems. For implementations as a servo mechanism, AC motors are electronically commutated and, as such, do not have brushes. While AC asynchronous motors have system acceleration limitations comparable to DC motor operated machines, permanent magnet AC synchronous motors eliminate brushes and have very high torque to inertia ratios by comparison and can, in principle, significantly increase dynamic response capabilities of the pitch actuator.
Existing AC system motor controls require power switching of IGBTs or MOSFETs in a three phase power bridge format, electronics and sensors to measure position and to commutate the multi phase power delivered to the rotating or static motor. Microprocessors are used to execute the commutation function and deliver switching or current control commands to the three-phase bridge so that the driven electric field has the appropriate angle with respect to the motor's magnetic field.
FIG. 2 is a schematic block diagram of a prior art brushless motor blade control system. For the sake of simplicity, not all pitch system functions or elements are shown in FIG. 2. The main drawback for a typical AC motor actuator system for pitch control is its substantial weaknesses from a safety system standpoint. For example, the normal operation and the safety operation share the same relatively complex hardware and software leading to the following:
1. Exceedingly high parts count for the hardware resulting in inherently lower reliability for the individual blade safety system;
2. Undesirably high level of complexity of the safety circuits leading to loss of reliability;
3. Common mode failure potential for all blades in the microprocessor functionality, application software implementation, and third party editor and compiler software tools.
4. The safety system on each blade relies on the same numerous and delicate parts and semiconductors used in the electronics for normal control, inviting common mode failure from AC mains generator disturbances, grid supply transients, or from lightning strikes. A systemic common mode failure caused by external electrical transients can result in a safety system failure on more than one blade.
5. Nearly any single point failure results in a failure of the safety system on at least one blade. While safety failure of one blade does not result in a catastrophic failure, such failure results in extraordinary impulse and fatigue conditions. These condition may offset the benefits of higher load mitigation capability, since the load case for the preceding condition must be accounted for at potentially higher frequency over the turbine design life than with the DC pitch system.